Realized Gains from Block-Plot Coastal Douglas-Fir Trials in the Northern Oregon Cascades

Realized gains for coastal Douglas-fir (Pseudotsuga menziesii var. menziesii) were evaluated using data collected from 15-year-old trees from five field trials planted in large block plots in the northern Oregon Cascades. Three populations with different genetic levels (elite - high predicted gain; intermediate - moderate predicted gain; and unimproved - wild seedlot) were compared at two planting spacings (1.8 × 1.8 m and 3.6 × 3.6 m). The realized gains at age 15 averaged over both the elite and intermediate progeny were 17.2% for stand volume per hectare, 3.5% for mean height, and 4.3% for diameter, compared to predicted genetic gains of 16.0% for volume, 5.4% for height, and 6.4% for diameter. Realized and predicted gains correlated well at the family level, with an average correlation coefficient close to 0.80. The improved populations also had higher survival rate and lower stem sinuosity than the unimproved population. Strong genetic level × planting spacing interaction effects were revealed for the growth traits at age 15 using mixed model analyses. Realized gains for stand per-hectare volume and mean growth rate were at least twice as large in the elite population as in the intermediate population at the close spacing. By contrast, both populations performed similarly at the wide spacing. This indicates that the selected genetic materials responded differently to the changes of competitive environment, and realized gain trials should closely mimic operational plantations in order to provide valid estimates of realized gains. Realized gains in per-hectare volume varied greatly among test sites. No significant genetic level x site interactions were found for any traits.